Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. Hip joint prostheses are common. The human hip joint acts mechanically as a ball and socket joint, wherein the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. Various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials. Prosthetic components are generally made from either metals, ceramics, or plastics.
Total hip arthroplasty and hemi-arthroplasty are two procedures well known within the medical industry for replacing all or part of a patient's hip. A total hip arthroplasty replaces both the femoral component and the acetabular surface of the joint, so that both a femoral prosthesis and an acetabular prosthesis are required. A conventional acetabular prosthesis may include a cup, a cup and a liner, or in some cases only a liner, all of which may be formed in various shapes and sizes. Generally, a metal cup and a polymeric liner are used. However, the liner may be made of a variety of materials, including polyethylene, ultra high molecular weight polyethylene and ceramic materials. The cup is usually of generally hemispherical shape and features an outer, convex surface and an inner, concave surface that is adapted to receive a cup liner. The liner fits inside the cup and has a convex and concave surface. The cup liner is the bearing element in the acetabular component assembly. The convex surface of the liner corresponds to the inner concave surface of the cup or acetabulum, and the liner concave surface receives the head of a femoral component. An acetabular cup may include a highly polished inner surface in order to decrease wear.
The liner concave surface, or internal concave surface, is characterized by features relative to an axis through the center of the concave surface. This axis may or may not be aligned with the central axis of the shell. In a typical liner the concave surface has a hemispherical geometry and is also referred to as the internal diameter. In such liners, the geometry is characterized by features that are concentric to an axis that runs through the center of the internal diameter.
An acetabular prosthesis may be fixed in the reamed acetabulum of a patient. Such a prosthesis may include a cup (or a cup and liner assembly) that is fixed either by placing screws through apertures in the cup or by securing the cup with cement. In some cases, only a liner is cemented in a patient due to poor bone stock. In other cases, a cup having a porous surface may be press fit into the reamed acetabular surface.
A femoral prosthesis used in total hip arthroplasty generally includes a spherical or near-spherical head attached to an elongate stem with a neck connecting the head and stem. In use, the elongate stem is located in the intramedullary canal of the femur and the spherical or near-spherical head articulates relative to the acetabular component. Femoral prostheses used in total hip arthroplasty procedures may or may not differ from an endoprosthesis used in a hemi-arthroplasty, described below. However, the femoral head of each type prosthesis is generally a standard size and shape. Various cups, liners, shells, stems and other components may be provided in each type arthroplasty to form modular prostheses to restore function of the hip joint.
Hemi-arthroplasty refers to replacing part of a hip joint, such as replacing a femoral component so that a femoral prosthesis articulates against natural body tissue in the patient's acetabulum. A femoral prosthesis implanted during a hemi-arthroplasty is generally referred to as an endoprosthesis. Generally, an endoprosthesis includes a stem, a head, and may include additional components such as shells and liners. Current endoprosthesis designs include (1) monoblock; (2) two-component; (3) three-component; and (4) five-component designs. A monoblock endoprosthesis is a one-piece structure including a femoral stem and head. Polarity refers to the number of articulating surfaces a prosthesis contains. A monoblock endoprosthesis has one articulation surface between the head and the patient's natural acetabulum, and is therefore referred to as monopolar.
A two-component endoprosthesis includes a femoral component and a shell. The femoral component may include a modular head and stem. A two-component design may be bipolar, so that the head articulates relative to the shell and the shell articulates relative to the acetabulum. A three-component endoprosthesis includes a femoral component, a liner, and a shell. Similar to a two-component design, the femoral component may include a modular head and stem. A three-component endoprosthesis may either be: bipolar, in which the liner is fixed in the shell; or tripolar, in which the head articulates relative to the liner, the liner articulates relative to the shell, and the shell articulates relative to the acetabulum. A five-component endoprosthesis includes a femoral component (which may include a modular head and stem), a first liner, a first shell, a second liner, and a second shell. Both of the first and second liners are fixed inside each of the first and second shells. Therefore, this design is a tripolar design: the second shell is free to articulate with respect to the acetabulum, the modular head of the femoral component articulates with respect to the first liner and the first shell articulates relative to the second liner. Thus, endoprostheses may be described both with respect to the number of components and with respect to the number of articulating surfaces as installed in a patient. Some current designs may also include a mechanical device, such as a snap-ring, for constraining the femoral head, further described below.
Endoprostheses, as well as total hip prostheses, may also be described as constrained and non-constrained prostheses. Non-constrained prostheses rely on the downward force of the body through the joint and the tension created by the soft tissue, including the muscles, ligaments and tendons, to retain the prosthesis in its implanted position. Other prostheses include mechanisms for preventing dislocation of the components, such as the implant stem head. Typically, these prostheses have restraint mechanisms that result in a smaller range of motion of the hip joint, and are generally referred to as “constrained” components.
One example of a restraint mechanism is a shell or liner having greater than hemispherical coverage around the head such that the head is constrained within the internal diameter, thus preventing subluxation and dislocation. In contrast to standard liners, constrained liners employ an extended, elevated portion over a segment of the periphery of the liner internal diameter in order to increase coverage of the femoral head and thus reduce the likelihood of dislocation and aid in reduction of the head should subluxation occur. While use of a constrained components is generally not desirable due to resulting decreased range of motion, the use of constrained components may be beneficial in cases of tenuous stability in order to avoid dislocation. See e.g. T. Cobb, et al., The Elevated-Rim Acetabular Liner in Total Hip Arthroplasty: Relationship to Postoperative Dislocation, Journal of Bone and Joint Surgery, Vol. 78-A, No. 1, January 1996, pp. 80-86. However, constrained components have a reduction in the arc of motion to contact in the direction of the elevated lip segment, thus, there is a substantial loss of overall range of motion compared to a standard liner. An implant stem head constrained by a shell or liner may dislocate if the femoral component rotates beyond the range of motion permitted by the assembly. Dislocation may occur because the edge or lip of the liner or shell that retains the implant stem head acts as a fulcrum about which the femoral component pivots, thereby causing the implant stem head to dislocate from its position within the liner or shell of the prosthesis. Dislocation of a hip prosthesis is painful and often requires medical intervention.
Three component bipolar endoprostheses including polyethylene liners are known in the industry, and suffer from at least three major clinical problems. First, the vast majority of articulation occurs between the liner and the shell, and it is not uncommon to obtain almost no relative motion between the shell and the acetabulum. Second, there is often a considerable amount of polyethylene wear debris generated from the device due to fatigue loading of the liner. Finally, there is a lower limit to the size of the shell due to the need to incorporate a standard head size and an appropriately thick liner. Current solutions to these problems include a design having a ceramic shell, a ceramic head and a polyethylene snap ring, which locks the head in the shell. Such designs frequently lead to polyethylene wear and have a complex assembly. Another solution has been use of a unipolar monoblock device, which does not require a liner but which results in excessive wear of the acetabulum.
Thus, there exists a need for a prosthetic component capable of retaining an implant stem head to prevent it from dislocating while providing a larger range of motion than is allowed by conventional constrained prostheses. There is also a need for a prosthetic component capable of retaining an implant head to prevent it from dislocating while eliminating the requirement of an inner bearing surface, or liner.